1. Field of the Invention
The present invention relates to a method of forming a transparent conductive film on a substrate of a color filter for liquid crystal displays and a transparent conductive film formed by the method.
2. Prior Art
Oxide indium films with addition of tin (hereinafter referred to "ITO films") are generally used as excellent transparent conductive films (transparent electrode films) formed on substrates of color filters for liquid crystal displays.
Conventionally known methods for forming such ITO films include a vacuum evaporation method, spattering method, and RF (radio frequency) ion-plating method.
Transparent conductive films for color liquid crystal displays are required to be thin for high resolution and high light transmittance, and also required to have a low specific resistance and high uniformity for increased size and increased response speed.
In forming an ITO film on a surface of a transparent glass plate or a transparent resin plate as a substrate of a color filter for a color liquid crystal display using any of the above methods, in order to avoid thermal deterioration of the color filter which is usually formed of an organic resin such as an epoxy resin or an acrylic resin with a pigment or a dye mixed therein, the formation of the ITO film is carried out with the temperature of the substrate being set to 250.degree. C. or less at or below which the deterioration of the resin forming the color filter does not occur. As a consequence of the low temperature of the substrate, the reaction of indium and oxygen on the substrate and crystallization of a film to be formed are not effected to a sufficient degree, resulting in formation of a film with a small crystal grain size and many defects, i.e. pores. The ITO film thus formed has many defects or pores which capture the carriers, and consequently the film has a reduced carrier electron density and hence an increased specific resistance.
To overcome the above disadvantage, there has been proposed an apparatus which forms an ITO film using an ion plating method. An example of this apparatus is shown in FIG. 1. In the figure, a vacuum vessel 1 inside which a vacuum chamber 1a is defined has a mounting opening 2 formed in a side wall thereof, at which a guide part 3 is mounted on the vacuum vessel 1. Mounted on the guide part 3 is an arc discharge plasma gun 4 such as a pressure gradient type plasma gun, which serves as discharge plasma generating means constituting a cathode. Further provided on the guide part 3 is a steering coil 5 for guiding a plasma beam. The plasma gun 4 includes a first intermediate electrode 6 and a second intermediate electrode 7 which are concentrically arranged for the convergence of the plasma beam.
The plasma gun 4 further includes an insulating tube 8, the interior of which communicates with a passage defined by the first and second intermediate electrodes 6, 7. Arranged inside the insulating tube 8 is a Mo cylinder 9 formed of molybdenum (Mo), inside which is arranged a Ta pipe 10 formed of tantalum (Ta), with a space between the cylinder 9 and the pipe 10 being partitioned by an annular plate 11 formed of LaB.sub.6 (lanthanum hexaboronite). Mounted on ends of the insulating tube 8, Mo cylinder 9 and Ta pipe 10 is a conductive plate 12 which has a carrier gas inlet opening 13 formed therein, through which an Ar gas as a carrier gas is introduced and passes through the Ta pipe 10.
At an upper location within the vacuum chamber 1a, a substrate 14 as an object to be processed is supported by a conveyer device 15. At a lower location within the vacuum chamber 1a, a hearth 17 which accommodates a permanent magnet 24 is arranged in opposed relation to the substrate 14 to serve as a main anode. An evaporation material 18 formed of indium oxide with addition of tin is received in the hearth 17. A magnet case 20 accommodating a permanent magnet 19 is arranged around the hearth 17 via an insulating material, not shown. The permanent 19 and the magnet case 20 constitute an auxiliary anode for correcting the plasma beam direction.
A negative electrode side of a variable voltage power supply 21 is connected to the conductive plate 12. A positive electrode side of the variable voltage power supply 21 is connected to the first intermediate electrode 6 through a resistor R1, as well as to the second intermediate electrode 7 through a resistor R2. The positive electrode side of the power supply 21 is also connected to the hearth 17, and grounded through a resistor R3.
A gas inlet opening 22 and a gas discharge opening 23 are formed in another side wall of the vacuum vessel 1. The gas inlet opening 22 introduces a carrier gas formed of a mixture of argon and oxygen or oxygen. The gas discharge opening 23 discharges the carrier gas within the vacuum chamber 1a to the outside.
With the above construction of the conventional ion plating apparatus, when the carrier gas is introduced through the gas inlet opening 22, a discharge occurs between the first intermediate electrode 6 and the Mo cylinder 9 so that a plasma beam 30 is generated. The plasma beam 30 is guided by the steering coil 5 and the permanent magnet 19 within the magnet case 20 to reach the hearth 17 forming the anode and the magnet case 20. Accordingly, the evaporation material 18 received in the hearth 17 is Joule-heated by the plasma beam 30 to evaporate. Particles of the evaporated material 18 are ionized while passing the plasma beam 30 and attached to a surface of the substrate 14 that is opposed to the hearth 17 to form a thin film (ITO film) thereon.
FIG. 2 is a vertical sectional view showing details of the substrate 14 with the ITO film formed on the surface thereof. As shown in the figure, a color filter 14b for a color liquid display is formed on a transparent glass plate 14a, and a protective film 14c formed of an organic resin such as an acrylic resin is formed on the color filter 14b. The color filter 14b is formed of an organic resin such as an acrylic resin with a pigment and a dye mixed therein.
In forming an ITO film which is a transparent conductive film using the conventional ion plating apparatus constructed as above, when the evaporation material 18 is heated by the plasma beam 30 to evaporate, particles of the evaporated material 18 are ionized while passing the plasma beam 30 and attached to the substrate 14 to form a thin film on the surface thereof. Since these particles of the evaporated material 18 are indium atoms, positively ionized indium particles attached to the substrate 14 react with an O.sub.2 component of the carrier gas introduced into the vacuum chamber 1a through the gas inlet opening 22 to form the ITO film 14d on the surface of the substrate 14. On this occasion, a vertical magnetic field is formed by the permanent magnet 19 and the magnet case 20 so that the plasma density increases to increase the temperature of electrons, which promotes the reaction of indium and O.sub.2 component of the carrier gas and crystallization thereof. Consequently, the formed ITO film has a sufficient carrier electron density and hence a reduced specific resistance. That is, an ITO film having a specific resistance of 150 .mu..OMEGA..multidot.cm or less can be obtained.
According to the conventional ion plating apparatus, however, the discharge voltage of the plasma beam 30 that reaches the hearth 17 is so high that positive ions generated when particles of the evaporated material 18 pass the plasma beam 30 are accelerated to a higher degree than required. Consequently, the accelerated particles are implanted into the ITO film being formed on the surface of the substrate 14 at too high a speed, to form defects or pores in the ITO film, resulting in increased film compressive stress (increased internal compressive stress of the ITO film). For example, an ITO film formed at 250.degree. C. or less has a film compressive stress of 0.55 GPa or more at a specific resistance of 230 .mu..OMEGA..multidot.cm or less, which is larger than that obtained by other film forming methods such as the vacuum evaporation method, spattering method, and RF ion-plating method.
Particularly, the manufacture of a color liquid crystal display essentially requires a step of heating a substrate formed of a resin for a color filter after an ITO film is formed on a surface of the substrate. During this step, the ITO film which has a high film compressive stress can become broken.